1. Field of the Invention
The process concerns a distillation system in which a water-free ethanol of a technical quality, usable as a fuel component for motor vehicle gasoline, can be obtained from crude ethanol in a particularly energy-saving manner. Additionally, an alternative of the process consists of using just a portion of the ethanol produced as a fuel component, while the predominant portion, also applicable as a fuel component, is quite definitely applicable for other purposes owing to its high degree of purity.
2. Description of the Prior Art
It is known from German Pat. No. 22 48 841 to remove water from isopropyl alcohol (which is similar to ethanol in many of its physical, chemical and fuel-technical properties) obtained by synthetic production from propylene and water in a two-column distillation system for fuel quality, and, to obtain it in a technical quality whereby the by-products of the synthesis are essentially not separated. The diisopropylether, which develops as a by-product of the synthesis, builds up initially in the dehydration column and is released with the produced water-free isopropylalcohol in the ratio of its development, serving in this instance as the entrainer during the dehydration. The separation of water is effected in the rectification column, which is operated under a higher pressure than the dehydration column, from the aqueous layer of the decanter for the head product of the dehydration column so that the heat content of the vapors of the rectification column can be used for the heating of the dehydration column.
It is, in a way, obvious to transfer this process to ethanol. However, in contrast to diisopropylether which forms a ternary azeotrope with isopropylalcohol and water with a boiling point of 62.degree. C. and a composition of 4.5% water, 4.5% isopropylalcohol and 91% ether, diethylether does not form a ternary azeotrope with ethanol and water; the binary azeotrope boiling at the lowest level in the presence of diethylether at normal pressure consists of 1.3% water and 98.7% ether, has a boiling point of 34.15.degree. C. and cannot be separated into two layers. Therefore, a separation of water and the return of a reflux with below azeotropic composition are not possible.
Theoretically, it would be possible to separate the water overhead with diethylether by applying a higher than atmospheric pressure in the dehydration column, since, with increasing pressure, the binary azeotropes of diethylether and water contain more water so that, finally, a separation into two phases is effected. However, a highly energy-saving distillation into a water-free ethanol is only possible when the vapors of the rectification column can be utilized for the heating of the dehydration column which presupposes a certain pressure drop between these two columns. The pressure difference must be selected in such a fashion that the boiling temperature of the alcohol vapors of the first column is at least 6.degree. C. higher than the sump temperature of the second column, since, otherwise, the boilers of the second column must be of an uneconomically large size or heat utilization is no longer possible when the sump temperature of the second column is higher than the boiling temperature of the alcohol vapors.
An additional disadvantage of diethylether as the entrainer for the dehydration of ethanol lies in the fact that no ether develops as a by-product in the production of ethanol by the fermentation of carbohydrates. It should be noted that ethanol produced by fermentation has the greatest importance when used as a fuel component as compared to that produced synthetically from ethylene. Therefore, the technical development towards the production, in as energy-saving a manner as possible of water-free, but otherwise not particularly well-purified ethanol, has taken other directions than that of the use of diethylether as an entrainer. Thus, K. Misselhorn reports in Chemie-Ingenieur-Technik 53 (1981), page 49 of steam consumption of 2.5 kg/liter of ethanol, corresponding to 3.2 kg steam/kg ethanol, the dehydration being achieved by operation of the rectification column under pressure and the utilization of vapor heat in the dehydration column and the entrainer recovery column.
The same principle is also applied in the European Patent Application No. 00 11 147 directed especially to ethanol for fuel use; in this instance, proceeding from an 8.5% fermentation alcohol, a specific consumption of 2.06 kg/liter of ethanol, corresponding to 2.6 kg steam/kg ethanol, is reported and a water-free alcohol is obtained by means of a rather comprehensive heat exchange of all three columns.
A thorough alcohol purification permitting another application, other than that in the field of fuel, is not really taken into consideration. However, higher alcohols building up between feeding and product drainage are drained, extracted after having been cooled with water and added, after separation, as organic phase to the fuel component while the alcohol is recovered in the first column from the additionally supplied water with the appropriate use of energy.
Furthermore, the azeotropically boiling alcohol (95% by volume is indicated as the concentration) is not drained from the head of the rectification column but at about the 50th of 60 plates as a liquid. In order to discharge easily boiling impurities (acetaldehyde is mentioned by name), a portion of the head vapors of the rectification column is not led to the column as a return flow but directly added to the water-free fuel alcohol with 95% by volume ethanol. This is possible since the applied benzenedehydration supplies easily an alcohol with less than 0.1% water as a sump product but 0.5% water can be tolerated in the addition to the fuel.
Without this being said in so many words, the known processes are obviously based on the consideration that, since the feeding of the azeotropically boiling alcohol into the dehydration column with its lower than ethanol boiling impurities leads to its enrichment in the head of this column so that, since it is essentially a question of hydrophile substances, such as acetaldehyde and methanol, finally a separation is no longer effected into two phases, a discharge of these more easily boiling substances is supposedly necessary. However, only a portion of the easily boiling impurities can be separated with the help of the measure proposed there and thus kept away from the dehydration column, and this in accordance with the selected return flow ratio, so that a portion of the head product must be processed continuously or intermittently in an additional column which is not shown in European Patent Application No. 00 11 147.
If crude alcohols are to be processed from alcoholic fermentation with higher contents of low-boiling impurities, for example, on the basis of spent sulfite liquors or saccharification of wood with up to 3% methanol, the methanol separation requires considerable additional expense relative to apparatus and energy.
A need, therefore, continues to exist for an energy-saving method for the production of dehydrated ethanol.